1. Technical Field
This invention relates to technology for making rapid free-form tooling and more particularly to making such tooling more wear resistant with a technique that does not require melting of metal but does follow a computer model of the tooling.
2. Discussion of the Prior Art
Direct fabrication of near-net shapes in metal, from a computer model, typically involves melting and solidification, which can cause high residual stresses, undesirable phases, poor microstructures, rough surface finish, warpage and other problems. Several prior art technologies can fabricate near-net shapes of metal and do so directly or indirectly from a computer model. One such technology is to use a laser that builds up a part by laying down successive small laser-welded beads in a computer-controlled pattern. Another technique uses directed light fabrication, others selective laser sintering. Yet others have used thermal spray deposition to build up a part by spraying molten metal droplets onto any work-piece surface, using computer-controlled masking and/or intermediate machining operations to control the pattern of the deposited material for the successive sprayed layers. Still another approach is investment casting of a part using a wax or polymeric mold pattern that is created with a computer-based process such as stereo-lithography. Although such technologies have a potential to cut time and costs for rapid prototyping, there are inherent difficulties that have hindered widespread application of such prior technological methods. All of these prior technologies involve melting and solidification. Each new layer starts out molten, solidifies, and must eventually cool to room temperature. In addition to simple dimensional changes, the resulting thermal contraction can produce significant residual stresses in the completed part and may even cause thin parts to warp. Moreover, undesirable phases or microstructures may be formed, especially at interfaces between dissimilar metals.
An object of this invention is to provide more wear resistant rapid tooling by uniquely selecting a soft metal as a tooling base and adding thereto welded layers of a thermally conductive metal and a wear resistant metal, such addition not requiring the melting of any such metals during fabrication.
The invention, in one aspect, that meets the above object, is a method of directly making rapid prototype tooling from a computer model having a free-form shape, comprising: (a) machining a soft metal tooling base so as to contour at least one free-form surface in conformity with the computer model; (b) cold-gas dynamic spraying the contoured surface to form superimposed impact-welded metal particle layers, the layers consisting of at least one thermal management under-layer comprising primarily copper, and at least an outer wear resistant layer comprising at least primarily tool steel.